A number of chemicals of interest have been tested for mutagenicity using Salmonella typhimurium in a number of different protocols. The carcinogen furan, a potent inducer of mutations in the ras oncogene in mice, was originally reported nonmutagenic in Salmonella. We have tested furan, and a synthesized metabolite, in Salmonella and found weak mutagenic activity for both chemicals in the absence of exogenous metabolic activation only. Modifications will be made in the test protocol in an attempt to enhance the responses, and to help determine the specific metabolite(s) responsible for furan's mutagenic activity. HC blue 1 and 2 are both direct-acting mutagens but only HC blue 1 is a rodent carcinogen. The carcinogenic HC blue 1 is also a more potent mutagen than HC blue 2. In an attempt to determine the reason for the absence of a carcinogenic response with HC blue 2, the chemicals are being studied in bacterial strains lacking one of the nitroreductase enzymes. The mutagenicities of both chemicals require bacterial nitroreductase, and the depression of mutagenicity in nitroreductase deficient strains of Salmonella is not reversed by the addition of rat liver S-9.